When a grayscale original image (i.e., a continuous-tone image) is recorded on various objects, used is FM (Frequency Modulated) screening where grayscale representation is achieved by changing the number of equal size dots irregularly disposed (each of which is a minimum recordable unit of dot formation, and it is also called as a microdot). Especially in an image recording apparatus such as a printer using an inkjet mechanism whose dot recording resolution is comparatively low, FM screening is frequently used. However, if a flight direction of ink droplet from one outlet is varied, streak unevenness (line mura) often appears in FM screening.
In an image recording system, disclosed in Japanese Patent Application Laid-Open No. 2006-14271 (Document 1), for recording an image on a printing plate, dot centers are arranged uniformly and randomly in a matrix area which is to store a threshold matrix. For each color component, a halftone cell which serves as a unit for generation of a halftone dot is set around each dot center. Threshold values are set in the threshold matrix so that dot-like halftone dot areas grow from the plurality of dot centers in accordance with the shape of the halftone cells with increase in gray level of an original image. In recording a halftone image on the printing plate, gray values for one color component of respective pixels in the original image and the corresponding threshold values in the threshold matrix are inputted to a comparator to generate signal of halftone image and the halftone image is recorded on the printing plate by a head. In the image recording system, since the density of the dot centers arranged randomly is changed color by color, graininess in the printed image can be reduced.
A multi-pen printer disclosed in Japanese Patent Application Laid-Open No. 7-186411 (Document 2) has color pens and a black pen. In the color pens, a resolution is 300 dpi and a print width is ⅓ inch. In the black pen, a resolution is 600 dpi and a print width is ½ inch. Since the high performance black pen is used, frequent text and graphics represented by only black can be printed with high speed and high quality. Also in printing color graphics or the like, a large portion of color graphics content is represented with the black component and the black component is printed at a higher resolution. Thus, quality of whole color graphics is improved. Throughput of printing color graphics can be also improved.
As described above, in an image recording apparatus using an inkjet mechanism, there may be a case where a flight direction of ink is skewed due to fabrication error of an outlet for ejecting ink or the like, and dots are formed at positions misaligned from the intended dot formation positions on an object. And when the misaligned dots are continuous in a scan direction of the outlets, streak unevenness appears in the image. Also in the case where there is an outlet whose ink ejection amount is lower, dots smaller than the surrounding dots are continuous in the scan direction and therefore streak unevenness appears.
There is a technique, for prevention of such streak unevenness, where the number of outlets per unit length in a width direction parallel to a recording surface of the object and orthogonal to the scan direction is made large. However, increase of outlets causes increase of manufacturing cost of the image recording apparatus. In other words, both suppression of manufacturing cost of the apparatus and suppression of streak unevenness could not be achieved at the same time.